FIG. 1 depicts a schematic diagram of a portion of a typical wireless telecommunications system, which provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals 101-1 through 101-3) that are situated within a geographic region. The heart of a typical wireless telecommunications system is Wireless Switching Center ("WSC") 120, which might be known also as a Mobile Switching Center ("MSC") or Mobile Telephone Switching Office ("MTSO"). Typically, Wireless Switching Center 120 is connected to a plurality of base stations (e.g., base stations 103-1 through 103-5) that are dispersed throughout the geographic area serviced by the system and to the local and long-distance telephone offices (e.g. local-office 130, local-office 138 and toll-office 140). Wireless Switching Center 120 is responsible for, among other things, establishing and maintaining calls between wireless terminals and between a wireless terminal and a wireline terminal (e.g., wireline terminal 150), which wireline terminal is connected to Wireless Switching Center 120 via the local and/or long-distance networks.
The geographic area serviced by a wireless telecommunications system is divided into spatially distinct areas called "cells." As depicted in FIG. 1, each cell is schematically represented by one hexagon in a honeycomb pattern; in practice, however, each cell has an irregular shape that depends on the topography of the terrain surrounding the cell. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with Wireless Switching Center 120.
For example, when wireless terminal 101-1 desires to communicate with wireless terminal 101-2, wireless terminal 101-1 transmits the desired information to base station 103-1, which relays the information to Wireless Switching Center 120. Upon receipt of the information, and with the knowledge that it is intended for wireless terminal 101-2, Wireless Switching Center 120 then returns the information back to base station 103-1, which relays the information, via radio, to wireless terminal 101-2.
FIG. 2 depicts a block diagram of a first base station architecture in the prior art, which comprises one or more radios that are capable of transmitting outgoing signals via a transmit antenna ("T.sub.X ") and receiving incoming signals via a receive antenna ("Rx"). According to this architecture, there is only one transmit antenna per cell that transmits omni-directionally and only one receive antenna per cell that receives omni-directionally.
Each radio in this architecture receives one incoming carrier signal via the receive antenna and demodulates that carrier signal into one or more baseband signals in accordance with the particular access scheme employed (e.g., frequency-division multiple access, time-division multiple access, code-division multiple-access, etc.). The incoming baseband signals are then transmitted to wireless switching center 120. Analogously, outgoing baseband signals from wireless switching center 120 are modulated by the radio in accordance with the particular multiplexing scheme employed (e.g., frequency-division multiplexing, time-division multiplexing, code-division multiplexing, etc.) for transmission via the transmission antenna.
When wireless telecommunications system 100 is a terrestrial system, in contrast to a satellite-based system, the quality and availability of service is subject to the idiosyncrasies of the terrain surrounding the system. For example, when the topography of the terrain is hilly or mountainous, or when objects such as buildings or trees are present, a signal transmitted by a wireless terminal can be absorbed or reflected such that the signal quality is not uniform at the base station. The result is that a base station's receive antenna can receive a direct path signal and one or more reflected signals from the wireless terminal at disparate phases such that the signals destructively interfere. This phenomenon is widely known as multipath fading or fast fading or Rayleigh fading.
FIG. 3 depicts a block diagram of a second base station architecture in the prior art, which supports a technique known as N-way receive diversity to mitigate the effects of multipath fading. The base station architecture depicted in FIG. 3 comprises one or more radios that are capable of transmitting outgoing signals via a single transmit antenna, as in the architecture of FIG. 2, but also comprises N spatially-separate receive antennas ("Rx.sub.1 " through "Rx.sub.N "). Because multipath fading is a localized phenomenon, it is highly unlikely that all of the spatially-separated receive antennas will experience multipath fading at the same time. Therefore, if an incoming signal is weak at one receive antenna, it is likely to be satisfactory at one of the others. As is well-known in the prior art, a diversity combiner associated with the radios can combine N incoming signals, each from one of N receive antennas, using various techniques (e.g., selection diversity, equal gain combining diversity, maximum ratio combining diversity, etc.) to improve the reception of an incoming signal.
FIG. 4 depicts a block diagram of a third base station architecture in the prior art, which supports a technique for increasing the traffic capacity of the telecommunications system. This technique is known as "base station sectorization." In accordance with base station sectorization, the cell serviced by a base station is subdivided into M tessellated pie-slices, each of which comprises a 360.degree./M sector whose focus is at the base station. The base station architecture in FIG. 4 comprises M sets of radios and associated transmit and receive antennas, as shown, each of which operates independently of the others, except that the transmit and receive antennas associated with each sector are generally implemented so as to principally transmit into and receive from that sector.
The architecture in FIG. 4 is, however, disadvantageous because it requires more radios than necessary to support a given traffic capacity, which unnecessarily increases the cost of the base station. The same average traffic capacity can be accommodated with fewer radios if they are pooled, as depicted in FIG. 5.
FIG. 5 depicts a block diagram of a fourth base station architecture in the prior art, which supports receive diversity, sectorization, and radio pooling. The architecture comprises: a plurality of radios 501-1 through 501-Z, sniffer radio 502, switch matrix 503, and M sets of transmit and receive antennas 504-1 through 504-M, interconnected as shown. In accordance with this architecture, sniffer radio 502 scans all of the potential sectors and channels in search of incoming signals. When sniffer radio 502 detects an incoming signal from a given sector, it directs switch matrix 503 to route the incoming signals from that sector to an appropriate radio and to route the outgoing signals from that radio to the same sector. Because any radio can receive from and transmit to any sector, this architecture requires fewer radios to support the same average traffic capacity as the architecture in FIG. 4. This architecture is disadvantageous, however, in that it requires a sniffer radio and a complex M.times.(N+1)+2 by M.times.(N+1) switch matrix to be added to the base station.
Therefore, the need exists for a base station architecture that supports receive diversity, sectorization, and radio pooling and avoids some or all of the costs and disadvantages associated with architectures in the prior art.